This invention relates to optical elements, such as optical recording media, e.g., optical discs, to methods for producing such discs and to molding compositions for producing them. Articles such as optical discs have generally been made from materials such as polycarbonates or polymethylmethacrylates. In recent times attempts have been made to produce such articles from ring-opened polymers. In order to obtain reasonable optical clarity, it was necessary to hydrogenate such polymers subsequent to polymerization. However, such polymers have not proved entirely suitable for use in optical applications. For example, molded products of such polymers do not exhibit an optimal spectrum of properties, such as good birefringence, melt-flow properties, and related properties which may render them not as suitable for use in optical applications as are already accepted polymers, e.g., the polycarbonate polymers discussed above.
Polymers obtained by the ring-opening polymerization of cycloolefins are well known. For example, U.S. Pat. Nos. 4,136,247; 4,136,248; 4,136,249 and 4,178,424, all assigned to the B.F. Goodrich Company, describe such polymers and their preparation and each is incorporated herein by reference.
The ring-opening polymerization of cycloolefins produces unsaturated polycycloolefins. Polycycloolefins obtained from polycycloolefin monomers, i.e., monomers containing more than one ring structure, e.g., dicyclopentadiene, are of particular interest. Monomers such as dicyclopentadiene provide a 1,3-cyclopentene repeat structure in the polymer, which is obtained by a ring-opening polymerization and cannot be obtained by addition polymerization. These unsaturated polymers are known to be reactive (sulphur vulcanizable) and they are known to exhibit a profile of properties suitable for, e.g., automotive parts, such as decorative trim.
It is known that saturated hydrocarbon polymers, e.g., polypropylene and polyethylene, exhibit improved dielectric properties, hydrolytic stability, oxidative stability, and reduced water absorption when compared to polymers containing ester, amide, alcohol and other functional groups. The dielectric properties of such saturated polymers are desirable for electrical applications when used as insulators. The high oxidative stability of saturated hydrocarbon polymers renders them particularly desirable for applications in harsh environments, as does their hydrolytic stability. When unsaturated polymers are saturated, i.e., when saturated polymers are prepared from unsaturated polymers, the saturated polymers exhibit a dramatic improvement in oxidative stability. However, often that improvement is attained at the expense of a significant reduction in the heat distortion temperature for the saturated derivatives of the unsaturated hydrocarbon polymers. This reduction in heat distortion temperature may often render the polymers thermally inadequate for end-use in electrical and electro-optical systems, despite an improvement in oxidative stability.
Generally, saturated derivatives of ring-opened polymerized cycloolefins exhibit lower glass transition temperatures, and thus lower heat distortion temperatures than their unsaturated precursors. Hydrogenated polymers of certain cycloolefins have been employed, either in blends, or in particular applications as homopolymers or copolymers. For example, Japanese Kokai Patent No. 60 [1985]-26024 discloses hydrogenated "cracked" homopolymers of tetracyclododecene and its copolymers with bicyclic norbornene. These polymers are disclosed as being useful for optical materials having good transparency, water-proofness (low water absorption), and heat resistance, which renders them suitable for compact discs, video discs, computer discs, etc.. However, if the teachings of this document are employed to produce polymers disclosed therein, a product exhibiting an inferior spectrum of properties results. For example, high glass transition temperatures needed for certain applications cannot be obtained with these copolymers without sacrificing other properties. Moreover, in addition to the fact that tetracyclododecene is a relatively expensive monomer to make, the materials of this patent do not exhibit optimized properties.
Japanese Kokoku Patent No. Sho. 58 [1983]-43412 discloses hydrogenated homopolymers of dicyclopentadiene wherein the dicyclopentadiene is first "cracked" and polymerized, followed by hydrogenation. The resultant polymers are disclosed as having improved solvent resistance. Methods for hydrogenating the dicyclopentadiene polymers are provided and methods for polymerization are shown in the examples.
Thus, although attempts have been made to prepare optical materials with saturated polynorbornene-type polymers, the art still lacks such materials which exhibit an optimum spectrum of properties. There has been a continuing need for improvement.
U.S. Pat. No. 3,557,072 discloses nonhydrogenated polynorbornenes of general interest which may be of use in applications wherein a transparent polymer is desired.